Diesel exhaust is the exhaust gas produced by a diesel engine, plus any contained particulates. Its composition may vary with the fuel type, rate of consumption or speed of engine operation (e.g., idling or at speed or under load), and whether the engine is in an on-road vehicle, farm vehicle, locomotive, marine vessel, or stationary generator or other application.
Diesel exhaust causes lung cancer and other diseases such as asthma, and many premature deaths. Methods exist to reduce nitrogen oxides (NOx) and particulate matter (PM) in the exhaust.
Some countries have set a date to stop selling diesel vehicles, and some city centres will ban diesel cars.
Composition
The primary products of petroleum fuel combustion in air are carbon dioxide and water. The exhaust from this combustion also contains nitrogen and surplus oxygen which passes through the combustion without participating in any chemical reaction. The other components exist primarily from pyrosynthesis. While the distribution of the individual components of raw (untreated) diesel exhaust varies depending on factors like load, engine type, etc., the table below shows a typical composition.
The physical and chemical conditions that exist inside any such diesel engines under any conditions differ considerably from spark-ignition engines, because, by design, diesel engine power is directly controlled by the fuel supply, not by control of the air/fuel mixture, as in conventional gasoline engines. As a result of these differences, diesel engines generally produce a different array of pollutants than spark-ignition engines, differences that are sometimes qualitative (what pollutants are there, and what are not), but more often quantitative (how much of particular pollutants or pollutant classes are present in each). For instance, diesel engines produce one-twenty-eighth the carbon monoxide that gasoline engines do, as they burn their fuel in excess air even at full load.
However, the lean-burning nature of diesel engines and the high temperatures and pressures of the combustion process result in significant production of NOx (gaseous nitrogen oxides), an air pollutant that constitutes a unique challenge with regard to their reduction. While total nitrogen oxides from petrol cars have decreased by around 96% through the adoption of exhaust catalytic converters as of 2012, diesel cars still produce nitrogen oxides at a similar level to those bought 15 years earlier under real-world tests; hence, diesel cars emit around 20 times more nitrogen oxides than petrol cars. Modern on-road diesel engines typically use selective catalytic reduction (SCR) systems to meet emissions laws, as other methods such as exhaust gas recirculation (EGR) cannot adequately reduce NOx to meet the newer standards applicable in many jurisdictions.
Moreover, the fine particles (fine particulate matter) in diesel exhaust (e.g., soot, sometimes visible as opaque dark-colored smoke) has traditionally been of greater concern, as it presents different health concerns and is rarely produced in significant quantities by spark-ignition engines. These especially harmful particulate contaminants are at their peak when such engines are run without sufficient oxygen to fully combust the fuel; when a diesel engine runs at idle, enough oxygen is usually present to burn the fuel completely. From the particle emission standpoint, exhaust from diesel vehicles has been reported to be significantly more harmful than those from petrol vehicles.
Diesel exhausts, long known for their characteristic smells, changed significantly with the reduction of the sulfur content of diesel fuel, and again when catalytic converters were introduced in exhaust systems. Even so, diesel exhausts continue to contain an array of inorganic and organic pollutants, in various classes, and in varying concentrations (see below), depending on fuel composition and engine running conditions. +Diesel engine exhaust compositionsSpeciesAverage(Reif 2014)Konrad Reif (ed): Dieselmotor-Management im Überblick. 2nd edition. Springer Fachmedien, Wiesbaden 2014, . p. 171Average(Merker,Teichmann, 2014)Günter P. Merker, Rüdiger Teichmann (ed.): Grundlagen Verbrennungsmotoren. 7th edition. Springer Fachmedien, Wiesbaden 2014, ., Chapter 7.1, Fig. 7.1Diesel's first engine(Hartenstein, 1895)Sass, Friedrich (1962), Geschichte des deutschen Verbrennungsmotorenbaus von 1860 bis 1918 (in German), Berlin/Heidelberg: Springer, . p. 466(Khair, Majewski, 2006)(various sources)Volume percentage (Volume?)percentage Nitrogen (N2) 75.2% 72.1% - ~67 % - Oxygen (O2) 15% 0.7% 0.5% ~9 % - Carbon dioxide (CO2) 7.1% 12.3% 12.5% ~12 % - Water (H2O) 2.6% 13.8% - ~11 % -Carbon monoxide (CO) 0.043% 0.09% 0.1% - 100–500 ppmNitrogen oxides () 0.034% 0.13% - - 50–1000 ppmHydrocarbons (HC) 0.005% 0.09% - - -Aldehyde 0.001% n/a - - -Particulate matter(sulfate + solid substances)0.008%0.0008% - -1–30 mg·m−3
Chemical classes The following are classes of chemical compounds that have been found in diesel exhaust.
Class of chemical contaminant NoteAntimony compounds Toxicity similar to arsenic poisoningBeryllium compounds IARC Group 1 carcinogensChromium compounds IARC Group 3 possible carcinogensCobalt compounds Cyanide compounds Dioxins and dibenzofurans Manganese compounds Mercury compounds IARC Group 3 possible carcinogensNitrogen oxides 5.6 ppm or 6500 μg/m³Polycyclic organic matter, includingpolycyclic aromatic hydrocarbons (PAHs) Selenium compounds Sulfur compounds
Specific chemicals The following are classes of specific chemicals that have been found in diesel exhaust.
Chemical contaminant Note Concentration, ppmAcetaldehyde IARC Group 1 carcinogensAcrolein IARC Group 2A probable carcinogensAniline IARC Group 2A probable carcinogensArsenic IARC Group 1 carcinogens, endocrine disruptor Benzene IARC Group 1 carcinogens Biphenyl Mild toxicity Bis(2-ethylhexyl) phthalate Endocrine disruptor 1,3-Butadiene IARC Group 1 carcinogensCadmium IARC Group 1 carcinogens, endocrine disruptor Chlorine Byproduct of urea injection Chlorobenzene "[L]ow to moderate" toxicity Cresol§ Dibutyl phthalate Endocrine disruptor 1,8-Dinitropyrene Strongly carcinogenic Ethylbenzene Formaldehyde IARC Group 1 carcinogens Inorganic lead Endocrine disruptor Methanol Methyl ethyl ketone Naphthalene IARC Group 2B carcinogens Nickel IARC Group 2B carcinogens 3-Nitrobenzanthrone (3-NBA) Strongly carcinogenic 0.6-6.64-Nitrobiphenyl Irritant, damages nerves/liver/kidneys 2.2Phenol Phosphorus Pyrene 3532–8002Benzo(e)pyrene 487–946Benzo(a)pyrene IARC Group 1 carcinogen 208–558Fluoranthene IARC Group 3 possible carcinogens 3399–7321Propionaldehyde Styrene IARC Group 2B carcinogens Toluene IARC Group 3 possible carcinogens Xylene§ IARC Group 3 possible carcinogens § Includes all regioisomers of this aromatic compound. See ortho-, meta-, and para-isomer descriptions at each compound's article.
Regulation
United States To rapidly reduce particulate matter from heavy-duty diesel engines in California, the California Air Resources Board created the Carl Moyer Memorial Air Quality Standards Attainment Program to provide funding for upgrading engines ahead of emissions regulations. In 2008, the California Air Resources Board also implemented the 2008 California Statewide Truck and Bus Rule which requires all heavy-duty diesel trucks and buses, with a few exceptions, that operate in California to either retrofit or replace engines in order to reduce diesel particulate matter.
European Union
Unlike international shipping, which had a sulfur limit of 3.5% mass/mass outside ECA until 2020, when it reduced to 0.5% outside ECA, diesel for on road use and off-road (heavy equipment) has been limited throughout the EU since 2009.
Diesel and gasoline have been limited to 10 ppm sulfur since 2009 (for on-road vehicles) and 2011 (non-road vehicles). Mandatory specifications also apply to more than a dozen fuel parameters.
Health damage
Damage to public health Emissions from diesel vehicles are more harmful than those from petrol vehicles. Diesel combustion exhaust is a source of atmospheric soot and fine particles, which is a component of the air pollution implicated in human cancer, heart and lung damage, and mental functioning. Moreover, diesel exhaust contains contaminants listed as carcinogenic for humans by the IARC (part of the World Health Organization of the United Nations), as present in their List of IARC Group 1 carcinogens.
Diesel exhaust is a Group 1 carcinogen, which causes lung cancer and has a positive association with bladder cancer. It contains several substances that are also listed individually as human carcinogens by the IARC.
A March 2025 report from the Centre for Research on Energy and Clean Air (CREA) reveals that the impact of diesel trucks is more harmful to public health than previously believed. CREA assessed the impacts of nitrogen oxide (NOX) emissions from medium and heavy-duty trucks manufactured and sold by Daimler, Traton, Volvo, and Paccar between 2014–2023. The analysis predicts that, through 2040, there could be 307,000 premature deaths, 217,000 new childhood asthma cases, 120 million lost workdays, and $1.4 trillion in economic costs linked to diesel truck emissions.
Occupational health effects
Exposure to diesel exhaust and diesel particulate matter (DPM) is an occupational hazard to truckers, railroad workers, occupants of residential homes in the vicinity of a rail yard, and miners using diesel-powered equipment in underground mines. Adverse health effects have also been observed in the general population at ambient atmospheric particle concentrations well below the concentrations in occupational settings.
In March 2012, U.S. government scientists showed that underground miners exposed to high levels of diesel fumes have a threefold increased risk for contracting lung cancer compared with those exposed to low levels. The $11.5 million Diesel Exhaust in Miners Study (DEMS) followed 12,315 miners, controlling for key carcinogens such as cigarette smoke, radon, and asbestos. This allowed scientists to isolate the effects of diesel fumes.
For over 10 years, concerns have been raised in the US regarding children's exposure to DPM as they ride diesel-powered school buses to and from school. In 2013, the Environmental Protection Agency (EPA) established the Clean School Bus USA initiative in an effort to unite private and public organizations in curbing student exposures.
Due to particulates
Diesel particulate matter (DPM), sometimes also called diesel exhaust particles (DEP), is the particulate component of diesel exhaust, which includes diesel soot and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates. When released into the atmosphere, DPM can take the form of individual particles or chain aggregates, with most in the invisible sub-micrometer range of 100 nanometers, also known as ultrafine particles (UFP) or PM0.1.
The main particulate fraction of diesel exhaust consists of fine particles. Because of their small size, inhaled particles may easily penetrate deep into the lungs. The polycyclic aromatic hydrocarbons (PAHs) in the exhaust stimulate nerves in the lungs, causing reflex coughing, wheezing and shortness of breath. The rough surfaces of these particles makes it easy for them to bind with other toxins in the environment, thus increasing the hazards of particle inhalation.